Various measures have been taken to store plants (e.g., fruits and vegetables) in a container used, e.g., in marine transportation for a long period of time while keeping the plants fresh. As an example of such measures, a technique for keeping freshness of plants, while taking the fact into account that freshness of plants considerably decreases as they breathe, has been known. In this technique, gas having a lower oxygen concentration than the outside air is supplied into the container to decrease the oxygen concentration of the air in the container, thereby reducing the breathing of the plants and keeping the plants fresh (see, e.g., Patent Document 1 below).
According to Patent Document 1, while the air in the container is cooled, an adsorbent column filled with a nitrogen adsorbent is used to produce nitrogen-enriched air having a lower oxygen concentration and a higher nitrogen concentration than outside air, and this nitrogen-enriched air is supplied into the container to reduce the oxygen concentration of the air in the container, making it easier to keep the plants fresh.